Since charged particles deposit most of their range in a sharp Bragg peak, charged-particle radiotherapy is ideal for the treatment of deep- seated tumors or tumors situated near critical normal structures. In heterogeneous media, however, physical processes such as multiple scattering can cause significant deterioration of the Bragg peak and can compromise the dose-localizing potential of charged-particle beams. In the proposed investigations, we will use Marte Carlo techniques to study multiple-scattering effects in simulated patient geometries, determine optimal beam-line configurations to minimize these effects, adn device a truly three-dimensional dose calculation algorithm which reflects the degradation in the Bragg peak due to multiple scattering in heterogeneous media. To accomplish these goals, a charged-particle Monte Carlo code incorporating multiple scattering and fragmentation processes will be developed. In tihs code, any volume will be represented as a collection of small volume elements (voxels), allowing relatively complicated geometries to be described. In particular, it will be possible to utilize CT data directly. The Monte Carlo code will be verified through benchmark measurements in simple phantoms. The ultimate goal of this research is to use calculated Monte Carlo data to develop and evaluate new, truly three-dimensional, dose-calculation models for charged-particle radiotherapy.